Olympiad Maths Prep

Track / Stage 5 / 204 of 400 #804 of 2000

Problem 804

AIME late
Geometry Difficulty 5.5 Find the answer

Question 144, As shown in the figure, plane ABDEABDE \perp plane ABC\mathrm{ABC}, ABC\triangle \mathrm{ABC} is an isosceles right triangle, AC=BC=4A C = B C = 4, quadrilateral ABDE\mathrm{ABDE} is a right trapezoid, BD//AE,BDAB,BD=2,AE=4\mathrm{BD} / / \mathrm{AE}, \mathrm{BD} \perp \mathrm{AB}, \mathrm{BD} = 2, \mathrm{AE} = 4, points OO and M\mathrm{M} are the midpoints of CE\mathrm{CE} and AB\mathrm{AB} respectively. Find the sine value of the angle formed by line CD\mathrm{CD} and plane ODM\mathrm{ODM}.

Official solution

Question 144, Solution: As shown in the figure, according to the conditions, we can supplement the figure to form a cube EFGHANBCA N B C with a side length of 4. Taking the midpoint of ACA C as KK, then OKO K is parallel and equal to BDB D, so quadrilateral ODBKO D B K is a rectangle. Taking the midpoint of AKA K as B2\mathrm{B}_{2}, then ML//BK//0D\mathrm{ML} / / \mathrm{BK} / / 0 \mathrm{D}.

Since SOLC=LCOK2=3×22=3S_{\triangle O L C}=\frac{L C \cdot O K}{2}=\frac{3 \times 2}{2}=3, and the distance from DD to the plane OLCO L C is 4, thus COOLD=3×43=4\mathrm{CO}-\mathrm{OLD}=\frac{3 \times 4}{3}=4. Also, OL=5,OD=25,LD2=29\mathrm{OL}=\sqrt{5}, \mathrm{OD}=2 \sqrt{5}, \mathrm{LD}^{2}=\sqrt{29}, so according to Heron's formula, SOLD=26\mathrm{S}_{\triangle \mathrm{OLD}}=2 \sqrt{6}. Therefore, we have
4=VCOLD=26d(C,OLD)3d(C,OLD)=6 4=V_{C-O L D}=\frac{2 \sqrt{6} \cdot d(C, O L D)}{3} \Rightarrow d(C, O L D)=\sqrt{6}

Noting that CD=25C D=2 \sqrt{5}, the sine value of the angle formed by the line CDC D and the plane ODMO D M is 625=3010\frac{\sqrt{6}}{2 \sqrt{5}}=\frac{\sqrt{30}}{10}.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.